**Calculating the Molality of a Solution**

To calculate the molality of a solution, we need to consider the mass of solute and the mass of solvent.

**Definition of Molality**

Molality (m) is defined as the number of moles of solute per kilogram of solvent. It is expressed in mol/kg.

**Given Data**

Density of 3M NaCl solution = 1.25 g/mL

**Step 1: Calculating the Mass of Solvent**

To calculate the mass of solvent, we need to know the volume of the solution and the density of the solution.

Given the density of the solution is 1.25 g/mL, we can assume 1 mL of the solution to have a mass of 1.25 g.

Since we have a 3M solution, this means that 1 L (1000 mL) of the solution contains 3 moles of NaCl.

Therefore, the mass of 1 L of the solution can be calculated as:

Mass of 1 L solution = density × volume
Mass of 1 L solution = 1.25 g/mL × 1000 mL
Mass of 1 L solution = 1250 g

**Step 2: Calculating the Mass of Solute**

Since we know that 1 L of the solution contains 3 moles of NaCl, we can calculate the mass of NaCl in 1 L of the solution using its molar mass.

The molar mass of NaCl is 58.44 g/mol.

Mass of solute = moles × molar mass
Mass of solute = 3 mol × 58.44 g/mol
Mass of solute = 175.32 g

**Step 3: Calculating the Mass of Solvent**

Now that we have the mass of the solute (NaCl) and the mass of the solution (1250 g), we can calculate the mass of the solvent (water) by subtracting the mass of the solute from the mass of the solution.

Mass of solvent = Mass of solution - Mass of solute
Mass of solvent = 1250 g - 175.32 g
Mass of solvent = 1074.68 g

**Step 4: Calculating the Molality**

Finally, we can calculate the molality of the solution by dividing the moles of solute by the mass of the solvent in kilograms.

Molality (m) = moles of solute / mass of solvent (in kg)
Molality (m) = 3 mol / 1.07468 kg
Molality (m) ≈ 2.79 mol/kg

Therefore, the molality of the 3M NaCl solution is approximately 2.79 mol/kg.

Molality = n/solvent
3 molar solution means 3 moles of Nacl solute is present in 1lit of solution .
weight of solute = 3× 58.5 (as gram molecular weight of Nacl is 58.5)
=175.5 gr
given density = 1.25
m/v =1.25
m/1000ml=1.25
m=1250gr
weight of the solvent = wgt of solution - wgt of solute
= 1250- 175.5
=1074.5
molality = 3×1000/1074.5 = 2.79 m